Our analysis of published cytogenetic data indicates that much mammalian recombination, especially in males, is not mediated by recombination nodules ("RNs"), structures that do account for most recombination in a wide variety of other organisms. Moreover, we detect a pattern: non-RN- recombination is not as randomly distributed along chromosomes as is RN- recombination; it is highly localized, producing recombinational "hot spots" in males near chromosome ends and in certain interstitial locations. We propose to use three approaches to study patters of RN- vs. non-RN- mediated recombination in mammalian (primarily human) spermatocytes and oocytes: 1) EM mapping of recombinational DNA repair synthesis at RN-and non-RN sites along synaptonemal complexes (SCs), visualized by immunogold detection of biotin-11-dUTP; 2) EM localization of particular sequences (telomere repeats, minisatellites and STIRS, or subtelomeric interspersed repeats) implicated as recombinational "hot spots" (again by biotin/immunogold detection) on pachytene and diakinesis/metaphase I bivalents, in order to determine which, if any, such sequences correspond to actual sites of recombination; 3) EM localization, by immunogold-labeled antibodies, of proteins implicated in mammalian meiotic recombination, in order to determine whether they are associated with RN- or non-RN-recombination. These molecular cytogenetic approaches will allow us to examine multiple sites of recombination at one time, differentiate between the two recombinational types, assess the relative importance of each in males and females, and determine which enzymes are involved in each type of recombination. This will allow us to ascertain whether differential expression of the non-RN- pathway accounts for the higher frequency of nondisjunction in females with it consequence of maternally derived aneuploidy.